This invention relates to plasma sources and, more particularly, to sources of soft X-ray or extreme ultraviolet photons, wherein high power production of photons is achieved by electrostatic acceleration of ions toward a plasma discharge region, neutralization of the ions to avoid space charge repulsion as the discharge region is approached and the application of a heating current through the central plasma in order to raise its temperature and density.
Soft X-ray and extreme ultraviolet photons can be generated in a hot plasma. The wavelength of the photons is determined by the mixture of ionization states present, with generally shorter wavelength photons being produced by the radiation of higher ionization states within the plasma. An example relevant to lithography is the xenon plasma that contains states Xe10+, Xe11+ and Xe12+ and radiates strongly in the 10-15 nanometer (nm) band of the spectrum. Within this band, the 13.5 nanometer wavelength is considered to be the optimum for lithography because it can be reflected with up to 70% efficiency by molybdenum-silicon multilayer mirrors in a combination that re-images the pattern of a semiconductor circuit from a mask onto a silicon wafer.
Several approaches to the generation of these energetic photons have been researched in recent years. The plasma has been heated by laser pulses in the so-called laser-produced-plasma (LPP) method. Also, the plasma has been heated directly by the passage of a pulsed electric current in a variety of discharge-produced plasma (DPP) photon sources. These include the capillary discharge, the dense plasma focus and the Z-pinch. It is believed that a viable 13.5 nm source for commercial, high throughput lithography will be required to emit approximately 100 watts of photon power into 2 steradians in a 2% fractional band at 13.5 nm, from a roughly spherical source of diameter less than 1.5 millimeters. In xenon, which is the most efficient 13.5 nm radiator (of room temperature gaseous elements), the 2% fractional band is produced at an electrical efficiency of approximately 0.5% into 2xcfx80 steradians in DPP sources and up to 1% into 2xcfx80 steradians in LPP sources relative to laser power absorbed. For the lithography source, a plasma power of 30-60 kilowatts (kW) is therefore required. Other requirements are for precise plasma positioning, to provide uniform illumination, and a repetition frequency greater than 6 kHz.
In the prior art, the plasma has been positioned, in the case of a laser produced plasma, by the intersection of a stabilized beam of liquid xenon with a focused laser beam. The size and positional stability of the resulting plasma are compatible with the application, but with laser efficiencies of only 4% for the pulsed lasers of interest, an electrical input power of 750 kW to 1.5 megawatts is likely to be needed in order to generate 100 watts of 13.5 nm photons, making the economics of the LPP source very unfavorable.
By supplying electrical energy directly to the plasma, the DPP source can, in principle, have a power input not much greater than the 30-60 kW plasma power. However, in prior art discharges, the plasma has, with the exception of the dense plasma focus, been too large in at least one dimension, and the dense plasma focus itself depends on a closely positioned electrode, only a few millimeters distant from the plasma, to create a small, positionally stable plasma focus. There are limits to the plasma power that can be generated in such close proximity to a solid electrode, presenting a difficult scaling challenge for the dense plasma focus source.
Pending application Ser. No. 09/815,633 filed Mar. 23, 2001 discloses a new photon source, referred to herein as the astron source, wherein energy and material are fed into a plasma at a central location via numerous energetic neutral beams. In this source, a relatively large separation has been achieved between the plasma and the nearest solid surface. The astron source also has a distributed electrode which exhibits low current density and anticipated longer life. Although this approach has enabled the generation of a hot plasma that emits extreme ultraviolet photons and is capable in principle of being scaled to 30-60 kW plasma power, it depends on a high acceleration efficiency for the neutral beam particles. To date, only 20% efficiency has been measured, and improvements in acceleration efficiency are required in order to give this photon source a good electrical efficiency.
Accordingly, there is a need for improved methods and apparatus for generating soft X-ray or extreme ultraviolet photons.
According to a first aspect of the invention, a source of photons comprises a discharge chamber, first and second groups of ion beam sources in the discharge chamber, and a neutralizing mechanism. Each of the ion beam sources electrostatically accelerates a beam of ions of a working gas toward a plasma discharge region. The first group of ion beam sources acts as a cathode, and the second group of ion beam sources acts as an anode for delivering a heating current to the plasma discharge region. The neutralizing mechanism at least partially neutralizes the ion beams before they enter the plasma discharge region. The neutralized beams and the heating current form a hot plasma in the discharge region that radiates photons.
The first and second groups of ion beam sources may comprise concentric electrode shells having sets of apertures aligned along axes which pass through the plasma discharge region. An inner electrode shell is divided into two half shells. The inner electrode half shells receive a voltage pulse that causes the heating current to be delivered to the plasma discharge region through channels formed by the ion beams. Electrical energy is delivered efficiently to the central plasma, causing heating and radiation from the ionic species of interest.
In one embodiment, the source includes a transformer having a secondary with one terminal coupled to the first group of ion beam sources and a second terminal coupled to the second group of ion beam sources. The primary of the transformer is coupled to a pulsed voltage source.
According to another aspect of the invention, a system is provided for generating photons. The system comprises a housing defining a discharge chamber, first and second groups of ion beam sources in the discharge chamber, a first voltage source for applying an accelerating voltage to the first and second groups of ion beam sources, a second voltage source for supplying a heating current through a plasma discharge region between the first and second groups of ion beam sources, a gas source for supplying a working gas to the discharge chamber, a neutralizing mechanism, and a vacuum system for controlling the pressure of the working gas in the discharge chamber. The ion beam sources each electrostatically accelerate a beam of ions of a working gas toward the plasma discharge region. The first group of ion beam sources acts as a cathode and the second group of ion beam sources acts as an anode for supplying the heating current through the plasma discharge region. The neutralizing mechanism at least partially neutralizes the ion beams before they enter the plasma discharge region, wherein the neutralized beams enter the plasma discharge region and form a hot plasma that radiates photons. The heating current heats and compresses the plasma, raising its temperature and density.
According to a further aspect of the invention, a method is provided for generating photons. The method comprises the steps of electrostatically accelerating a plurality of beams of ions of a working gas toward a plasma discharge region, at least partially neutralizing the ions beams before they enter the plasma discharge region and supplying a heating current through the plasma discharge region, wherein the neutralized beams and the heating current form a hot plasma that radiates photons.
According to a further aspect of the invention, a source of photons comprises a discharge chamber, a plurality of ion beam sources in the discharge chamber, a neutralizing mechanism and an external electrode. The ion beam sources each electrostatically accelerate a beam of ions of a working gas toward a plasma discharge region. The neutralizing mechanism at least partially neutralizes the ion beams before they enter the plasma discharge region. The external electrode is spaced from the plasma discharge region and delivers a heating current to the plasma discharge region, wherein the neutralized beams and the heating current form a hot plasma that radiates photons.